The present invention relates to a system using a power converter controlled by switching, for example, a system that drives various loads such as a motor by the output of a power converter.
A power converter controlled by switching is used for driving various equipment such as a motor and is also utilized in fields of household electrical appliances, transportation equipment, an uninterruptible power system, a solar generator and a fuel cell. To realize required performance, the improvement of the switching characteristics of power elements forming the power converter is required to be promoted, switching speed for pulse width control is 10 to 100 ns and is considerably fast.
When pulse voltage having an abrupt waveform and generated because of high speed PWM (Pulse Width Modulation) control is applied to equipment connected to the power converter, for example, an AC reactor connected to the side of a converter (the side of an alternating-current power supply) and a load (a motor and others) connected to the side of an inverter, the pulse voltage is reflected at each terminal of the AC reactor and the load and an oscillatory microsurge is caused. The terminal voltage of the AC reactor and the load rises because of the constant of wiring between the power converter and the AC reactor or the load.
When the microsurge is caused, the insulation of equipment forming a system is deteriorated, and the reliability and the life are deteriorated. Besides, in case common mode current via the stray capacitor of the equipment is caused because of surge voltage and a load is a motor, shaft current that flows on the side of the shaft of the motor is caused. The generation of the shaft current particularly comes into question in case an applied object is an electric railcar, an iron and steel plant and others. Further, as strong radiation noise is caused in the vicinity of the power supply terminal of the load, electromagnetic interference is caused in near electronic equipment and a near information communication device. Particularly, as most objects that utilize a recent power converter use information equipment, the effect is serious. Besides, the effect upon wireless LAN being popularized and networked home by wireless to be expected in future is also serious.
FIG. 11 shows the schematic configuration of a motor driving system which is one example of a system using a power converter controlled by switching. The motor driving system shown in FIG. 11 includes an AC power supply 1, an AC reactor 2, a power converter 3 and a motor 4. The AC reactor 2, a frame (not shown) of the motor 4 and the earth line (not shown) of the power converter 3 are grounded. The frame denotes structure which supports equipment and which covers the whole equipment with a conductor in a state in which the structure is electrically insulated from a current-carrying part of the equipment.
The power converter 3 receives power supplied from the AC power supply 1 via the AC reactor 2 and converts it to power of an arbitrary frequency and arbitrary voltage. The power converter 3 basically includes a converter (a power rectifier) 31 that converts input ac power to dc voltage, a smoothing capacitor 32 that smoothes dc voltage output from the converter 31 and an inverter (a power inverter) 33 that converts the smoothed dc voltage to ac voltage, these components are mounted on a wiring board (not shown), and the converter 31 and the inverter 33 are connected via dc main circuit conductors 30n, 30p formed on the wiring board. A cooling fin (not shown) is attached to each element case including the converter 31 and the inverter 33. The cooling fin is provided to suppress the temperature rise of the elements and is electrically connected to an earth line (not shown) of the wiring board.
In such a system, pulse voltage having an abrupt waveform and generated at the terminal of the inverter 33 because of high speed PWM (Pulse Width Modulation) control is propagated to the terminal of the motor 4 via a cable 34 and is reflected at the terminal of the motor 4 because the impedance is unconformable. Therefore, at the terminal of the motor 4, a microsurge is caused. Similarly, pulse voltage having an abrupt waveform and generated at the terminal of the converter 31 is propagated to the terminal of the AC reactor 2 via a cable 35, is reflected at the terminal of the AC reactor 2 because the impedance is mismatched, and a microsurge is also caused at the terminal of the AC reactor.
FIGS. 12A to 12E show voltage waveforms at the terminals of the inverter 33 and the motor 4. FIG. 12A shows a voltage waveform at the terminal of the inverter 33 and FIG. 12B shows an enlarged waveform of a part. FIG. 12C shows a voltage waveform at the terminal of the motor 4 and FIG. 12D shows an enlarged waveform of a part. FIG. 12E shows a measured example of the voltage waveform at the terminal of the motor 4. As shown in FIGS. 12A to 12E, a microsurge is remarkable on the side of the terminal of the motor 4, and it is known that the surge becomes large as the cable 34 is extended.
FIGS. 13A to 13E show voltage waveforms at the terminals of the converter 31 and the AC reactor 2. As in FIGS. 12A to 12E, FIG. 13A shows a voltage waveform at the terminal of the converter 31 and FIG. 13B shows an enlarged waveform of a part. FIG. 13C shows a voltage waveform at the terminal of the AC reactor 2 and FIG. 13D shows an enlarged waveform of a part. FIG. 13E shows a measured example of the voltage waveform at the terminal of the AC reactor 2. As shown in FIGS. 13A to 13E, a microsurge is also remarkable on the side of the terminal of the AC reactor 2.
For a measure against the deterioration of the reliability and the life by the microsurge, there is a method of increasing the withstand voltage of the motor. For a method of suppressing a microsurge, there are a method of using a microsurge suppression filter (refer to a patent document 1), a method of equalizing the impedance of the motor and the cable (refer to a non-patent document 1), a method of inserting a damping circuit at the terminal of the motor (refer to a non-patent document 2) and a method of inserting a high-tension circuit into a control system of the power converter and delaying a leading edge of voltage (refer to a non-patent document 3).
However, as the proper performance of the power converter may be deteriorated by the addition of these means for the measures, countermeasures in which wavelike effect upon the whole system including the power converter is fully examined are required. Besides, as countermeasures in which the characteristics of the cables and the input impedance and others of the motor and others are fully grasped are required, measurement requiring labor and intricate computing are required and improvement is required in terms of flexibility.
Further, when anew component is added for a measure against a microsurge, the induction of common mode current from the component and the generation of radiation noise cannot be avoided. Besides, when the capacity of the power converter is increased, the large-sizing of the component for a countermeasure which is made of iron and which includes winding structure and the increase of the weight cannot be avoided and the cost is increased.
[Patent Document 1]
JP-A-2002-58162
[Non-Patent Document 1]
A. F. Moreira, T. A. Lipo, G. Venkataramanan, and S. Bernet, “High-Frequency Modeling for Cable and Induction Motor Overvoltage Studies in Long Cable Drives”, IEEE Trans. Ind. Applicat., vol. 38, pp. 1297-1306, September/October 2002.
[Non-Patent Document 2]
N. Aoki, K. Satoh, and A. Nabae, “Damping Circuit to Suppress Motor Terminal overvoltage and Ringing in PWM Inverter-Fed AC Motor Drive Systems with Long Motor Leads”, IEEE Trans. Ind. Applicat., vol. 35, pp. 1014-1020, September/October 1999.
[Non-Patent Document 3]
Hasegawa, Domoto, and Akagi, “Design and Characteristics of Three-phase Sinewave Voltage Output PWM Inverter System Passive EMI Filter That Generates No Common Mode Voltage”, Electrical Theory D, vol. 122, No. 8, pp. 845-852, 2002.